Serrated trailing edge panel for a wind turbine blade

ABSTRACT

A serrated panel ( 70 ) for a wind turbine blade is disclosed. The panel ( 70 ) is configured to be attached to the trailing edge of a blade to form a plurality of serrations ( 71 ) at the trailing edge of the blade. The serrated panel comprises a base part ( 72 ) for attaching the panel ( 70 ) to the trailing edge of the blade. An exterior surface ( 78 ) of the base part comprises a corrugated surface in direction between longitudinal ends of the panel such that the exterior surface comprises crests ( 82 ) aligned substantially with midpoints of bases ( 80 ) of the serrations ( 71 ) and valleys ( 83 ) aligned substantially between serrations ( 71 ).

This is a Continuation Application of U.S. patent application Ser. No.16/072,939, filed Jul. 26, 2018, a National Phase Application filedunder 35 U.S.C. 371 as a national stage of PCT/EP2017/052640, filed Feb.2, 2017, and an application claiming the benefit of European ApplicationNo. 16155507.3, filed Feb. 12, 2016, the content of each of which ishereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to a serrated trailing edge panel for awind turbine blade as well as a wind turbine blade provided with such aserrated trailing edge panel.

BACKGROUND OF THE INVENTION

Wind power is increasingly popular due to its clean and environmentallyfriendly production of energy. The rotor blades of modern wind turbinescapture kinetic wind energy by using sophisticated blade design createdto maximise efficiency. However, complaints have arisen about thecreation of noise associated with the operation of wind power plants.Hence, noise reducing devices and associated blade designs areincreasingly desired.

One of the continued considerations for wind turbine blade design is theoperational noise produced when such blades rotate, in particular foron-shore wind turbine installations. A portion of such noise isgenerated from airflow leaving the profile of a wind turbine blade atthe blade trailing edge, and is often referred to as trailing edgenoise.

As modern wind turbine blades are manufactured at increasingly longerblade lengths, the longer span of the blades results in higher relativewind speeds experienced by the blades. Accordingly, this can lead torelatively large levels of trailing edge noise.

To this end, modern wind turbine blades are sometimes provided withserrations along the blade trailing edges, in an effort to reduce bladetrailing edge noise and/or to improve wind turbine blade efficiency, ascan be seen in EP1314885. The serration are typically provided byattaching a serrated panel at the trailing edge of the wind turbineblade. While the noise mitigating properties of such serrations areadvantageous, several drawbacks remain. Often, finding the rightserration geometry is a trade-off between noise mitigation performanceand structural requirements. This may lead to the need for thicktrailing edge regions, especially near the serration base. Thisincreased thickness has been found by the present inventions to become apotential source of additional noise.

Further, the panels may be rather stiff, which may lead to increasedpeeling forces and stress concentrations going into the laminatestructure of the blade, in particular at the longitudinal ends of thepanel.

There are several ways of attaching a panel to the trailing edge of theblade. US 2011/0142635 A1 describes a blade extension panel providedwith a cutaway mounting portion for mounting the extension panel to theblade such that the blade extension is substantially flush with asurface of the blade. The cutaway portion defines a notch configured tolocate the blade extension relative to the rotor blade. However, such anembodiment entails that the cutaway portion must be specificallydesigned for a specific portion of a wind turbine blade.

Accordingly, there is still a need to optimise the design of suchserrations or trailing edge serration panels in order to maximise noisemitigation.

It is therefore an object of the invention to provide a wind turbineblade having an improved trailing edge configuration as well as atrailing edge serration panel for arrangement at the trailing edge of awind turbine blade.

It is another object of the invention to provide a wind turbine bladedesign having serrations as well as a trailing edge serration panel,which design provide improved noise reduction during operation.

SUMMARY OF THE INVENTION

According to the invention, there is provided a serrated panel for awind turbine blade, wherein the panel is configured to be attached tothe trailing edge of a blade to form a plurality of serrations at thetrailing edge of the blade, wherein the serrated panel comprises:

-   -   a base part for attaching the panel to the trailing edge of the        blade, the base part having        -   a first longitudinal end for arrangement nearest a tip end            of the wind turbine blade,        -   a second longitudinal end for arrangement nearest a root end            of the wind turbine blade,        -   a first side for arrangement nearest a leading edge of the            blade,        -   a second side for arrangement farthest from the leading edge            of the blade,        -   an attachment surface for attaching to an exterior surface            of the wind turbine blade, and        -   an exterior surface facing away from the exterior surface of            the wind turbine blade, when the serrated panel is attached            to the wind turbine blade, and    -   a plurality of serrations extending from the second side of the        base part, wherein the serrations comprise a base proximal to        second side of the base part, and an apex distal to the second        side of the base part with a notional line extending from a        midpoint of the base to the apex.

According to a first aspect, the exterior surface of the base partcomprises a corrugated surface in direction between the firstlongitudinal end and the second longitudinal end such that the exteriorsurface comprises crests aligned substantially with the midpoints ofbases of the serrations and valleys aligned substantially betweenserrations.

This design makes it possible to decrease the thickness of the panel andreduce the longitudinal stiffness, which in turn leads to decreasedpeeling forces and stress concentrations going into the blade laminate,in particular at the longitudinal ends of the panels. The thin parts,i.e. the valleys, of the panel, result in less force transferred to thepanel when subjected to blade straining, e.g. from blade deflections.The thick parts, i.e. the crests, of the panel provide higher stiffnessto the serrations so as to ensure them to withstand aerodynamic loadingand prevent fluttering. In addition hereto, the corrugated design hassurprisingly been found to provide more efficient noise mitigation thanserrated panels having a uniform thickness in the longitudinaldirection.

According to a second aspect, the serrations comprises a shape and aremutually spaced so that a slit is formed between adjacent serrations,wherein each of the slits comprises a first side wall at a firstadjacent serration and a second side wall at a second adjacent serrationand a connecting surface extending between the first side wall and thesecond side wall, wherein the connecting surface is substantially flator flattened and comprises a first rounded surface at the first sidewalland a second rounded surface at the second side wall.

In other words, the slit does not have a fully rounded or semi-circularend section near the base part of the serrated panel, but comprises aflattened surface with rounded portions at the sidewalls. The flattenedpart may have a radius of curvature (and thus form part of circularpath). However, the radius of curvature of the flattened part is largerthan the radius of curvature of the first rounded surface and the secondround surface. Such a design has surprisingly been found to furtherreduce stress between serrations compared to conventional serratedpanels, and thus lower the risk of damages to the serrated panel, whenthe wind turbine blade bends due to pressure fluctuations.

It is clear that the connecting surface is arranged proximal to thesecond side of the base part and that the first sidewall and secondsidewall extend in a direction away from the second side of the basepart.

According to a third aspect, the serrated panel comprises two discretealignment protrusions that protrude from an attachment side of theserrated panel, the two discrete alignment notches being configured toabut a trailing edge of the wind turbine blade and thereby align theserrated panel relative to the trailing edge of the blade. The increasedreliability of positioning the serrated panel correctly ensures lessnoise defects due to misalignments, which in turn ensures a betteroverall noise reduction performance.

Thereby, a serrated panel is provided which may easily be aligned withthe trailing edge and will facilitate an easier and quicker mounting ofthe panels. The use of exactly two discrete alignment protrusionsensures that the panel fits to all sections of the trailing edgeirrespective of the shape and curve of the trailing edge of the blade.

According to a fourth aspect, the invention provides a wind turbineblade having a profiled contour including a pressure side and a suctionside, and a leading edge and a trailing edge with a chord having a chordlength extending therebetween, the wind turbine blade extending in aspanwise direction between a root end and a tip end, the wind turbineblade comprising at least one serrated panel according to any of thepreceding aspects and provided along at least a portion of the trailingedge of the blade. The at least one serrated panel may be any of thepanels according to the first, the second or the third aspects or be acombination thereof.

In the following a number of advantageous embodiments will be described.The embodiments may be applied to any of the first, the second, thethird, and the fourth aspects of the invention or combinations thereof.

According to a first embodiment, the base part comprises a maximumthickness line extending in a direction between the first longitudinalend and the second longitudinal end, the maximum thickness line definingthe position, where the serrated panel in a transverse cross-sectionalview has a maximum thickness. The maximum thickness line may bepositioned with a spacing from the first side of the base part.

In one advantageous embodiment, the base part is tapered from themaximum thickness line towards the first side of the base part. Thisprovides a smooth transition to the surface of the blade.

The base part may for instance comprise substantially triangular surfacethat extend from valleys at the first side of the base part and to acrest at the maximum thickness line.

In another advantageous embodiment, the serrations, and optionally thebase part, are tapered from the maximum thickness line towards theapexes of the serrations. This provides a gradual transition towards theapex of the serrations, which has advantages both in relation tostiffness transition and noise mitigation.

The crests advantageously extend from the base part of the serrationsand along the notional line of the serrations.

In an advantageous embodiment, a ratio between a thickness of the crestand a thickness of the valley is at least 3:2 and preferably at least2:1. In another advantageous embodiment, the ratio between a thicknessof the crest and a thickness of the valley is at most 5:1.

In yet another embodiment, the first sidewall and the second sidewallcomprises a section, where the first sidewall and the second sidewallare substantially parallel.

In one embodiment, the connecting surface has a curvature of radius(R_(b)), which is larger than those (R₁,R₂) of the first rounded surfaceand the second rounded surface. R_(b) is advantageously at least 5times, more advantageously at least 7 times, and even moreadvantageously at least 10 times larger than R₁ and R₂. If theconnecting surface is straight, R_(b) approaches infinity.

In one embodiment, the two discrete alignment protrusions are arrangedon the attachment surface of the base part. Thereby, the alignmentprotrusions may be arranged to very accurately align the attachment partof the serrated panel to the trailing edge of the blade.

In another embodiment, the two discrete alignment protrusions arearranged near the second side of the base part. Thereby, it is ensuredthat only the serrations themselves extend from the trailing edge of theblade.

In yet another embodiment, a first discrete protrusion is arranged nearthe first longitudinal end of the base part and a second discreteprotrusion is arranged near the second longitudinal end of the basepart. Thereby, it is ensured that the serrated panel may extend alongand substantially parallel to a large part of the trailing edge.

In principle, the protrusions may also be arranged on the serrations,preferably near the base of the serrations.

The serrated panels may be made in a polymer material, such aspolyurethane or polycarbonate, or from a fibre-reinforced polymermaterial.

In an advantageous embodiment, the serrated panels comprises at leasttwo serrations extending from the second side of the base part, moreadvantageously at least three, four or five serrations.

In an advantageous embodiment, the serrated panels comprises at most 20serrations extending from the second side of the base part, moreadvantageously at most 15, 12 or 10 serrations.

The invention also provides a wind turbine blade for a rotor of a windturbine having a substantially horizontal rotor shaft, the rotorcomprising a hub, from which the wind turbine blade extendssubstantially in a radial direction when mounted to the hub, the windturbine blade extending in a longitudinal direction parallel to alongitudinal axis and having a tip end and a root end,

the wind turbine blade comprising a profiled contour including apressure side and a suction side, as well as a leading edge and atrailing edge with a chord having a chord length extending therebetween,the profiled contour, when being impacted by an incident airflow,generating a lift,

wherein the wind turbine blade further comprises at least one serratedpanel according to any of the aforementioned embodiments provided alongat least a portion of the trailing edge of the blade.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a wind turbine;

FIG. 2 shows a schematic view of a wind turbine blade according to theinvention;

FIG. 3 shows a schematic view of an airfoil profile of the blade of FIG.2;

FIG. 4 shows a schematic view of the wind turbine blade of FIG. 2, seenfrom above and from the side;

FIG. 5A shows a bottom view of a serrated panel of the wind turbineblade;

FIG. 5B shows a side view of the serrated panel of FIG. 5A;

FIG. 5C shows a top view of the serrated panel of FIG. 5A;

FIG. 5D shows an end view of the serrated panel of FIG. 5A; and

FIG. 5E illustrates a bottom view of a slit formed between adjacentserrations of the serrated panel of FIG. 5A.

It will be understood that elements common to the different embodimentsof the invention have been provided with the same reference numerals inthe drawings.

FIG. 1 illustrates a conventional modern upwind wind turbine 2 accordingto the so-called “Danish concept” with a tower 4, a nacelle 6 and arotor with a substantially horizontal rotor shaft. The rotor includes ahub 8 and three blades 10 extending radially from the hub 8, each havinga blade root 16 nearest the hub and a blade tip 14 furthest from the hub8, the blade extending in a spanwise direction between the root 16 andthe tip 14. The rotor has a radius denoted R.

FIG. 2 shows a schematic view of a wind turbine blade 10. The windturbine blade 10 has the shape of a conventional wind turbine blade andcomprises a root region 30 closest to the hub, a profiled or an airfoilregion 34 furthest away from the hub and a transition region 32 betweenthe root region 30 and the airfoil region 34. The blade 10 comprises aleading edge 18 facing the direction of rotation of the blade 10, whenthe blade is mounted on the hub, and a trailing edge 20 facing theopposite direction of the leading edge 18. An array of trailing edgeserrations are provided along a portion of the trailing edge 20 of theblade. In general, flow of air over the wind turbine blade 10 extendsfrom the leading edge 18 to the trailing edge 20 in a generallytransverse or chordwise direction. While the serrations in FIG. 2 aredepicted as being arranged along a outboard portion of the blade, it isrecognised that the serrations may be arranged for instance closer tothe root of the blade 10, or that they may be arranged along forinstance the entire airfoil region 34 of the blade 10.

According to the inventions, the serrations are provided in form ofserrated panels 70, 70′ that are attached to a surface of the blade 10at the trailing edge 20 of the blade 10. The serrated panels 70, 70′ maycomprise serration having different sizes. The serrations near the tipof the blade 10 may for instance as shown in FIG. 2 have a smallerdimensions than serrations closer to the root of the blade 10.

The airfoil region 34 (also called the profiled region) has an ideal oralmost ideal blade shape with respect to generating lift, whereas theroot region 30 due to structural considerations has a substantiallycircular or elliptical cross-section, which for instance makes it easierand safer to mount the blade 10 to the hub. The diameter (or the chord)of the root region 30 is typically constant along the entire root area30. The transition region 32 has a transitional profile 42 graduallychanging from the circular or elliptical shape 40 of the root region 30to the airfoil profile 50 of the airfoil region 34. The chord length ofthe transition region 32 typically increases substantially linearly withincreasing distance r from the hub.

The airfoil region 34 has an airfoil profile 50 with a chord extendingbetween the leading edge 18 and the trailing edge 20 of the blade 10.The width of the chord decreases with increasing distance r from thehub.

It should be noted that the chords of different sections of the bladenormally do not lie in a common plane, since the blade may be twistedand/or curved (i.e. pre-bent), thus providing the chord plane with acorrespondingly twisted and/or curved course, this being most often thecase in order to compensate for the local velocity of the blade beingdependent on the radius from the hub.

FIG. 3 shows a schematic view of an airfoil profile 50 of a typicalblade of a wind turbine depicted with the various parameters, which aretypically used to define the geometrical shape of an airfoil. Theairfoil profile 50 has a pressure side 52 and a suction side 54, whichduring use—i.e. during rotation of the rotor—normally face towards thewindward (or upwind) side and the leeward (or downwind) side,respectively. The airfoil 50 has a chord 60 with a chord length cextending between a leading edge 56 and a trailing edge 58 of the blade.The airfoil 50 has a thickness t, which is defined as the distancebetween the pressure side 52 and the suction side 54. The thickness t ofthe airfoil varies along the chord 60. The deviation from a symmetricalprofile is given by a camber line 62, which is a median line through theairfoil profile 50. The median line can be found by drawing inscribedcircles from the leading edge 56 to the trailing edge 58. The medianline follows the centres of these inscribed circles and the deviation ordistance from the chord 60 is called the camber f. The asymmetry canalso be defined by use of parameters called the upper camber (or suctionside camber) and lower camber (or pressure side camber), which aredefined as the distances from the chord 60 and the suction side 54 andpressure side 52, respectively.

Airfoil profiles are often characterised by the following parameters:the chord length c, the maximum camber f, the position d_(f) of themaximum camber f, the maximum airfoil thickness t, which is the largestdiameter of the inscribed circles along the median camber line 62, theposition d_(t) of the maximum thickness t, and a nose radius (notshown). These parameters are typically defined as ratios to the chordlength c. Thus, a local relative blade thickness t/c is given as theratio between the local maximum thickness t and the local chord lengthc. Further, the position d_(p) of the maximum pressure side camber maybe used as a design parameter, and of course also the position of themaximum suction side camber.

FIG. 4 shows some other geometric parameters of the blade. The blade hasa total blade length L. As shown in FIG. 2, the root end is located atposition r=0, and the tip end located at r=L. The shoulder 40 of theblade is located at a position r=L_(w), and has a shoulder width W,which equals the chord length at the shoulder 40. The diameter of theroot is defined as D. Further, the blade is provided with a pre-bend,which is defined as Δy, which corresponds to the out of plane deflectionfrom a pitch axis 22 of the blade.

The wind turbine blade 10 generally comprises a shell made offibre-reinforced polymer, and is typically made as a pressure side orupwind shell part 24 and a suction side or downwind shell part 26 thatare glued together along bond lines 28 extending along the trailing edge20 and the leading edge 18 of the blade 10. Wind turbine blades aregenerally formed from fibre-reinforced plastics material, e.g. glassfibres and/or carbon fibres which are arranged in a mould and cured witha resin to form a solid structure. Modern wind turbine blades can oftenbe in excess of 30 or 40 metres in length, having blade root diametersof several metres. Wind turbine blades are generally designed forrelatively long lifetimes and to withstand considerable structural anddynamic loading.

FIG. 5A shows a bottom view of serrated panel 70, FIG. 5B shows a sideview of serrated panel 70, FIG. 5C shows a top view of serrated panel70, FIG. 5D shows an end view of serrated panel 70, and FIG. 5Eillustrates a slit 85 formed between adjacent serrations 71 of theserrated panel 70.

The serrated panel 70 (also called a serrated trailing edge panel) isconfigured to be attached to the trailing edge 20 of the blade 10 toform a plurality of serrations 71 at and projecting from the trailingedge of the blade. The serrated panel 70 comprises a base part forattaching the panel to the trailing edge of the blade. The base part 72has a first longitudinal end 73 for arrangement nearest a tip end of thewind turbine blade, a second longitudinal end 74 for arrangement nearesta root end of the wind turbine blade, a first side 75 for arrangementnearest a leading edge of the blade, a second side 76 for arrangementfarthest from the leading edge of the blade, an attachment surface 77for attaching to an exterior surface of the wind turbine blade 10, andan exterior surface 78 facing away from the exterior surface of the windturbine blade 10, when the serrated panel 70 is attached to the windturbine blade 10.

The serrated panel 70 further comprises a plurality of serrations 71extending from the second side 76 of the base part 72. The serrations 71each comprise a base 79 proximal to second side 76 of the base part, andan apex 80 distal to the second side 76 of the base part 72 with anotional line 81 extending from a midpoint of the base 79 to the apex80.

According to a first aspect, the exterior surface 78 of the base partmay comprise a corrugated surface in direction between the firstlongitudinal end 73 and the second longitudinal end 74 such that theexterior surface comprises crests 82 aligned substantially with themidpoints of bases 80 of the serrations 71 and valleys 83 alignedsubstantially between serrations 71.

This corrugated design makes it possible to decrease the thickness ofthe panel 71 and reduce the longitudinal stiffness, which in turn leadsto decreased peeling forces and stress concentrations going into theblade laminate, in particular at the longitudinal ends 73, 74 of thepanels 70. The thin parts, i.e. the valleys 83, of the panel 70, resultin less force transferred to the panel when subjected to bladestraining, e.g. from blade deflections. The thick parts, i.e. the crests82, of the panel 70 provide higher stiffness to the serrations 71 so asto ensure them to withstand aerodynamic loading and prevent fluttering.In addition hereto, the corrugated design has surprisingly been found toprovide more efficient noise mitigation than serrated panels having auniform thickness in the longitudinal direction.

The corrugated surface may for instance be triangular or saw toothshaped, e.g. with rounded valleys and crests. The corrugated surface mayalso be substantially sinusoidal.

The base part 72 of the serrated panel 70 may further comprise a maximumthickness line 84 extending between the first longitudinal end 75 andthe second longitudinal end 76, wherein the maximum thickness line 84defines the position, where the serrated panel in a transversecross-sectional view has a maximum thickness. The maximum thickness linemay be positioned with a spacing from the first side 75 of the base part72, and the base part 72 may further be tapered from the maximumthickness line 84 towards the first side 75 of the base part 72, e.g. asshown in the side view in FIG. 5D. Thereby, a smooth transition to thesurface of the wind turbine blade may be achieved.

The base part 72 may as indicated in FIG. 5C comprise substantiallytriangular surface parts that extend from valleys 82 at the first side75 of the base part 72 and to a crest 83 at the maximum thickness line84.

The serrations 71 (and optionally the base part 72) may as indicated inthe side view in FIG. 5D tapered from the maximum thickness line 84towards the apexes 80 of the serrations 71.

The crests may as shown in FIG. 5C extend from the base part 76 of theserrations 71 and along the notional line 82 of the serrations 71.

As shown in FIG. 5B, the thickness of the crests is defined at t_(c) andthe thickness of the valleys is defined as t_(v). The ratio betweent_(c) and t_(v) (e.g. at the maximum thickness line 84) may be at least3:2 and advantageously at least 2:1. The serrations 71 mayadvantageously further have rounded edges 95.

According to a second aspect, the serrations 71 may comprise a shape andare mutually spaced so that a slit 85 is formed between adjacentserrations 71. FIG. 5E shows a detail of such a slit 85. Each of theslits 85 comprises a first side wall 86 at a first adjacent serrationand a second side wall 87 at a second adjacent serration and further aconnecting surface 88 extending between the first side wall 86 and thesecond side wall 87. The connecting surface 88 is substantially flat orflattened and comprises a first rounded surface 89 at the first sidewall86 and a second rounded surface 90 at the second side wall 87. In otherwords, the slit does not have a fully rounded or semi-circular endsection near the base part of the serrated panel, but comprises aflattened surface with rounded portions at the sidewalls. The flattenedpart may have a radius of curvature (and thus form part of circularpath). However, the radius of curvature of the flattened part is largerthan the radius of curvature of the first rounded surface and the secondround surface. Such a design has surprisingly been found to furtherreduce stress between serrations compared to conventional serratedpanels, and thus lower the risk of damages to the serrated panel, whenthe wind turbine blade bends due to pressure fluctuations.

In one embodiment, the connecting surface 88 has a curvature of radius(R_(b)), which is larger than those R₁,R₂ of the first rounded surfaceand the second rounded surface. R_(b) is advantageously at least 10times larger than R₁ and R₂.

According to a third aspect, the serrated panel 70 may be provided withtwo discrete alignment protrusions 91, 92 that protrude from anattachment side 72 of the serrated panel 70. The two discrete alignmentnotches may be configured to abut the trailing edge 20 of the windturbine blade 10 and thereby align the serrated panel 70 relative to thetrailing edge 20 of the blade 10. The two discrete alignment protrusions91, 92 may be arranged near the second side 76 of the base part 70. Thediscrete alignment protrusions are advantageously arranged near thefirst longitudinal end 73 of the base part 72 and the secondlongitudinal end 74 of the base part 72, respectively.

The serrations 71 of the serrated panel 70 may be angled relative to thebase part 72 of the serrated panel, such that the base part 72 and theserrations 71 form an angle α. The angle α may be zero degrees, in whichcase the serrated panel 70 is straight. For angled serrated panels, theangle α may be between 2 and 15 degrees, typically around 5 or 10degrees.

The invention is not limited to the embodiments described herein, andmay be modified or adapted without departing from the scope of thepresent invention.

The invention claimed is:
 1. A serrated panel (70) for a wind turbineblade, whereby the serrated panel (70) is configured to be attached to atrailing edge of a wind turbine blade to form a plurality of serrations(71) at the trailing edge of the wind turbine blade, wherein theserrated panel comprises: a base part (72) configured for attaching theserrated panel (70) to the trailing edge of the wind turbine blade, thebase part (72) having: a first longitudinal end (73) configured forarrangement nearest a tip end of the wind turbine blade; a secondlongitudinal end (74) configured for arrangement nearest a root end ofthe wind turbine blade; a first side (75) configured for arrangementnearest a leading edge of the wind turbine blade; a second side (76)configured for arrangement farthest from the leading edge of the windturbine blade; an attachment surface (77) configured for attaching to anexterior surface of the wind turbine blade; and an exterior surface (78)configured to face away from the exterior surface of the wind turbineblade when the serrated panel (70) is attached to the wind turbineblade; and wherein the plurality of serrations (71) extend from thesecond side (76) of the base part (72), wherein the serrations (71) eachcomprise a base (79) proximal to the second side (76) of the base part,and an apex (80) distal to the second side (76) of the base part (72)with a notional line (81) extending from a midpoint of the base (79) tothe apex (80), wherein the serrations comprises a shape and are mutuallyspaced so that a slit is formed between adjacent serrations, whereineach of the slits (85) comprises a first side wall (86) at a firstadjacent serration and a second side wall (87) at a second adjacentserration and a connecting surface (88) extending between the first sidewall (86) and the second side wall (87), wherein the connecting surface(88) is flattened and comprises a first rounded surface (89) at thefirst sidewall (86) and a second rounded surface (90) at the second sidewall (87).
 2. The serrated panel (70) according to claim 1, wherein thefirst sidewall and the second sidewall comprises a section, where thefirst sidewall and the second sidewall are substantially parallel. 3.The serrated panel (70) according to claim 1, wherein the connectingsurface has a curvature of radius (R_(b)), which is larger than acurvature of radius of the first rounded surface and a curvature ofradius of the second rounded surface.
 4. The serrated panel (70)according to claim 1, wherein the exterior surface (78) of the base partcomprises a corrugated surface in a direction between the firstlongitudinal end (73) and the second longitudinal end (74) such that theexterior surface comprises crests (82) aligned substantially with themidpoints of respective bases (80) of the serrations (71) and valleys(83) aligned substantially between serrations (71).
 5. The serratedpanel (70) according to claim 4, wherein the base part (72) comprises amaximum thickness line (84) extending in a direction between the firstlongitudinal end (75) and the second longitudinal end (76), the maximumthickness line (84) defining the position, where the serrated panel in atransverse cross-sectional view has a maximum thickness, a transversedirection being defined as a direction between the first side and thesecond side and transverse to a direction between the first longitudinalend and the second longitudinal end.
 6. The serrated panel (70)according to claim 5, wherein the maximum thickness line is positionedwith a spacing from the first side (75) of the base part (72).
 7. Theserrated panel (70) according to claim 6, wherein the base part (72) istapered from the maximum thickness line towards the first side (75) ofthe base part (72).
 8. The serrated panel according to claim 6, whereinthe base part comprises substantially triangular surface that extendfrom valleys at the first side (75) of the base part and to a crest atthe maximum thickness line.
 9. The serrated panel (70) according toclaim 5, wherein the serrations are tapered from the maximum thicknessline towards the apexes (80) of the serrations (71).
 10. The serratedpanel (70) according to claim 9, wherein the base part (72) is taperedfrom the maximum thickness line towards the apexes (80) of theserrations (71).
 11. The serrated panel (70) according to claim 4,wherein the crests extend from the base part (72) of the serrations (71)and along the notional line (82) of the serrations.
 12. The serratedpanel (70) according to claim 4, wherein a ratio between a thickness ofthe crest and a thickness of the valley is at least 2:1.
 13. Theserrated panel (70) according to claim 4, wherein the ratio between athickness of the crest and a thickness of the valley is at most 5:1. 14.The serrated panel (70) according to claim 1, wherein the serrated panel(70) comprises two discrete alignment protrusions (91, 92) that protrudefrom an attachment side of the serrated panel (70), the two discretealignment notches being configured to abut a trailing edge of the windturbine blade and thereby align the serrated panel (70) relative to thetrailing edge of the blade.
 15. The serrated panel (70) according toclaim 14, wherein a first discrete protrusion is arranged near the firstlongitudinal end of the base part and a second discrete protrusion isarranged near the second longitudinal end of the base part.
 16. A windturbine blade (10) having a profiled contour including a pressure sideand a suction side, and a leading edge (18) and a trailing edge (20)with a chord having a chord length extending therebetween, the windturbine blade (10) extending in a spanwise direction between a root endand a tip end, the wind turbine blade comprising at least one serratedpanel (70) provided along at least a portion of the trailing edge (20)of the wind turbine blade (10), wherein the at least one serrated panel(70) comprises: a base part (72) configured for attaching the at leastone serrated panel (70) to the trailing edge of the wind turbine blade,the base part (72) having: a first longitudinal end (73) configured forarrangement nearest a tip end of the wind turbine blade; a secondlongitudinal end (74) configured for arrangement nearest a root end ofthe wind turbine blade; a first side (75) configured for arrangementnearest a leading edge of the wind turbine blade; a second side (76)configured for arrangement farthest from the leading edge of the windturbine blade; an attachment surface (77) configured for attaching to anexterior surface of the wind turbine blade; and an exterior surface (78)configured to face away from the exterior surface of the wind turbineblade when the at least one serrated panel (70) is attached to the windturbine blade; and a plurality of serrations (71) extending from thesecond side (76) of the base part (72), wherein the serrations (71) eachcomprise a base (79) proximal to the second side (76) of the base part,and an apex (80) distal to the second side (76) of the base part (72)with a notional line (81) extending from a midpoint of the base (79) tothe apex (80), wherein the plurality of serrations (71) comprises ashape and are mutually spaced so that a slit is formed between adjacentones of the serrations, wherein each of the slits (85) comprises a firstside wall (86) at a first adjacent serration and a second side wall (87)at a second adjacent serration and a connecting surface (88) extendingbetween the first side wall (86) and the second side wall (87), whereinthe connecting surface (88) is flattened and comprises a first roundedsurface (89) at the first sidewall (86) and a second rounded surface(90) at the second side wall (87).
 17. A serrated panel (70) for a windturbine blade, whereby the serrated panel (70) is configured to beattached to a trailing edge of a wind turbine blade to form a pluralityof serrations (71) at the trailing edge of the wind turbine blade,wherein the serrated panel comprises: a base part (72) configured forattaching the serrated panel (70) to the trailing edge of the windturbine blade, the base part (72) having: a first longitudinal end (73)configured for arrangement nearest a tip end of the wind turbine blade;a second longitudinal end (74) configured for arrangement nearest a rootend of the wind turbine blade; a first side (75) configured forarrangement nearest a leading edge of the wind turbine blade; a secondside (76) configured for arrangement farthest from the leading edge ofthe wind turbine blade; an attachment surface (77) configured forattaching to an exterior surface of the wind turbine blade; and anexterior surface (78) configured to face away from the exterior surfaceof the wind turbine blade when the serrated panel (70) is attached tothe wind turbine blade; and wherein the plurality of serrations (71)extend from the second side (76) of the base part (72), wherein theserrations (71) each comprise a base (79) proximal to the second side(76) of the base part, and an apex (80) distal to the second side (76)of the base part (72) with a notional line (81) extending from amidpoint of the base (79) to the apex (80), wherein the serrationscomprises a shape and are mutually spaced so that a slit is formedbetween adjacent serrations, wherein each of the slits (85) comprises afirst side wall (86) at a first adjacent serration and a second sidewall (87) at a second adjacent serration and a connecting surface (88)extending between the first side wall (86) and the second side wall(87), wherein the connecting surface (88) is substantially flat andcomprises a first rounded surface (89) at the first sidewall (86) and asecond rounded surface (90) at the second side wall (87).